Each key in such a keyboard essentially consists of a key button with a magnetic member, typically of ferrite, or pin fastened therein, which when operated plunges or enters into windings arranged on a printed circuit board of the matrix-like structured keyboard and generates a coupling signal. It also includes means for eliminating the effects of inherent mechanical and electrical tolerances upon the adjustment of the triggering point.
Keyboards of the above-mentioned type have been in widespread use as part of a computer terminal. The adjustment of the triggering point or triggering level in this known keyboard is currently done by a two-stage compensation process. An IC-component (SN 7445) is used as a current switch, which requires a supply voltage of 5 volts plus or minus 5%. Because of procurement difficulties expected for the IC SN 7445, and in order to expand the tolerance of the supply voltage, this component has been replaced by seven P-N-P transistors. This, however, entails that the tolerance of the collector-emitter voltage of the current switches increases, and thus the tolerance of the triggering point is also increased. The tolerances are also increased when the number of windings in the coils is diminished, which allows a cost optimized fabrication of the printed circuit boards. These intended measures would, in a known keyboard, require at least a three-stage compensation for each individual row of keys for adjustment of the triggering point.
The tolerances which affect the triggering point or level of an inductive keyboard are caused by a great variety of influences, for instance, by the fabrication tolerances of the ferrite pins inserted into the key buttons, by the separate coupling tolerances of the secondary coils, by the tolerance of the offset voltage of a signal readout amplifier, by the different internal resistances of the individual signal switches in a multiplexer for the matrix, by temperature fluctuations and supply voltage tolerances, and by different current wave slopes of the electronic pulse generator due to different current switch thresholds.
A contactless, inductively functioning circuit arrangement for application in a keyboard arranged in a matrix-like manner s also known from the European patent publication EP 009 420, which provides a differential transformer for generation of the switching function. A differential transformer is assigned to each individual key, which essentially is formed from two pairs of primary-secondary windings, wherein the primary and secondary windings are respectively arranged in one plane and are interconnected. In order to change the mutual inductance, one component is moved from an external position into a position within one pair of primary-secondary windings, while no such component changing the mutual inductance is provided in the second pair of windings. Apart from the disadvantage of requiring a larger area, a compensation of the fundamental interlink signal tolerances is required for each key as well as for changes in the supply voltage